Factors leading to slope instability in the Ursa Basin, Gulf of Mexico continental slope

Author(s): Urgeles, Roger; Locat, Jacques; Flemings, Peter B.; Dugan, Brandon; Nguyen Thi Thanh Binh; Sawyer, Derek E.
Author Affiliation(s): Primary:
Universitat de Barcelona, Spain
Université Laval, Canada
University of Texas at Austin, United States
Rice University, United States
University of Tokyo, Japan
Volume Title: 33rd international geological congress; abstracts
Source: International Geological Congress [International Geological Congress, Abstracts = Congrès Géologique International, Résumés, Vol.33; 33rd international geological congress, Oslo, Norway, Aug. 6-14, 2008. Publisher:], [location varies], International CODEN: IGABBY
Note: In English
Summary: The Ursa Basin, at ∼1000 m depth on the eastern levee of the Mississippi Canyon, Gulf of Mexico continental slope, is an extraordinary natural laboratory to investigate large-scale aseismic slope failure phenomena. Recent studies have suggested that seismic activity recorded in this area, is a result, at least in part, of shallow gravitational sliding rather than deep-seated tectonic processes. Extensive occurrence of Mass Transport Deposits (MTDs) in the Ursa Basin, both in time and space is also documented by multibeam and seismic reflection data. In June 2005, Integrated Ocean Drilling Program (IODP) Expedition 308 drilled three Sites adjacent to major Recent failures and mud-volcano type fluid escape structures, and through a series of MTDs of Pleistocene to Holocene age. Along these holes a complete suite of logs, sedimentological and geotechnical data were acquired, which illuminate factors that control failure initiation, provide insights into the failure mechanism itself and allow characterization of the hazard potential from future slope instabilities. On seismic data most MTDs appear as transparent bodies, and this appears to indicate some degree of remobilization of the failure mass, yet they do not appear to have moved far from the failure initiation area. Little overburden was removed because the failed masses did not evacuate the failing zone. MTDs that appear as seismic transparent bodies do not exceed 60 to 70 m in thickness. The first 60-70 m of overburden in Ursa Basin are characterized by very high porosity, decreasing down hole from 80 to 55%, and water content, rapidly decreasing from 100 to 40%, i.e. from values near the liquid limit to very close to the plastic limit. From measurements of porosity and stress state, we infer that fluid overpressure, derived from rapid sedimentation, is the most likely factor that initiated/controlled slope instability in the past. Fluid overpressures result in effective stresses that are 50 to 70% lower relative to hydrostatic conditions. Isotropically consolidated, undrained triaxial tests suggest low cohesion and a friction angle around 28° for fine-grained mudstones. This indicates that, given the slope geometry, nearly lithostatic overpressures were/will be needed to trigger slope failure. Profuse evidence of fluid escape structures, including large mud volcanoes, near failure scarps might indicate that such conditions existed in the past.
Year of Publication: 2008
Research Program: IODP Integrated Ocean Drilling Program
Key Words: 07 Marine Geology and Oceanography; Atlantic Ocean; Cenozoic; Clastic rocks; Continental slope; Expedition 308; Failures; Friction angles; Geologic hazards; Geometry; Gravity sliding; Gulf of Mexico; Integrated Ocean Drilling Program; Lithostatic pressure; Mass movements; Mechanism; Mississippi Canyon; Mud volcanoes; Mudstone; Natural hazards; North Atlantic; Overpressure; Porosity; Quaternary; Sedimentary rocks; Shear strength; Slope stability; Triaxial tests; Ursa Basin; Water content
Coordinates: N271500 N280600 W0890000 W0942500
Record ID: 2012046108
Copyright Information: GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data supplied by International Geological Congress Organizational Committee